Add some tolerance to VoronoiDiagram tester

git-svn-id: http://svn.osgeo.org/geos/trunk@4008 5242fede-7e19-0410-aef8-94bd7d2200fb
Merge branch 'svn-trunk' into svn-trunk-fast-rect-intersection
Update expected results to structurally match overlay-op ones